The metal foam material considered and the manufacturing process are described in the international patent applications WO 2004089564 and WO 2005037467 respectively. According to this process, the metal foam is produced in sheets with the desired length and width.
Foam based filters to be used or contained in said devices, are known for diesel particulate filtering applications, although most of them are based on ceramic materials. Typically ceramic foams with sufficiently small pore sizes to achieve good filtration efficiencies exhibit a relatively high pressure drop, due to the large number of closed pores. Moreover, as more soot is accumulated in the filter, such foams are known to lower their filtration efficiency until a “blow-off” behavior, i.e. a negative efficiency, is observed.
Another drawback of the ceramic foams is the limitation regarding the shaping of the final filter to be used or contained in said devices. Therefore, it is very difficult to obtain a large filtration area in the limited space required in automotive applications.
There is known a metal catalyst carrier with in use, an exhaust of an internal combustion engine, which enters exhaust pipe and enters tube passageway and flows outwardly through perforations therein, and which is diverted out by an interior closure. An annular baffle redirects the gas flow through perforations back into passageway and through a gas-permeable plug catalyst member, then flowing through a further plug catalyst member and continues to be forced into a serpentine flow path in and out of tube passageway via perforations. This path forces the exhaust into contact with exhaust pipe catalytic coating and inner catalytic coating of conformable catalyst member. Surfaces of conformable catalyst member and the inner surface of exhaust pipe may be coated with catalytic materials. The catalytically contacted gas undergoing pollution abatement reactions by contacting the catalytic coatings, emerges as a cleaned exhaust. This engine however is substantially different from the device of the subject invention.
In the known prior art, the material proposed is generally referred to as porous metal, without specifically detailing the production process.
Besides, some known embodiments are defined by the need to support the thermal regeneration of the filters via the usage of electrical heaters. Therefore the usage of at least two cylindrical filter elements is called for. Due to the limitations related with the material, the production process of the filter elements and of the need of multiple filter elements, the thickness of the filter elements is restricted in the region 0.5 to 20 mm, preferably 1 mm or more.
The pore size range proposed to be used particularly preferably from 100 to 600 μm is fairly small. Similarly the filtration thicknesses are very narrow. Besides, a simple reference to a “3-D trapping effect” lacks to demonstrate an understanding of the actual filtration mechanisms.
In further known prior art, there are two elements to be considered as fundamental consisting of the so-called “porosity gradient” principle and use of catalytically coated foam material which are not addressed.
A still further known prior art relates to the use of ceramic foams resulting in that they do not provide a gradient porosity.